The mycelium infects the plant through wounds or the stomata but also directly through the cuticle after the formation of an infection cushion. Dispersal is by rain, water, machinery and tools and in soil particles and via movement of plant parts. The fungus grows from plant to plant and from the soil upward, therefore the lowest plant parts are infected first. Plants that are generally growing well are less susceptible. Optimum temperature for infection is between 15 and 18 °C, but infection still occurs at 35 °C. Disease is most severe in a moderately wet soil.
On the infected plant parts, mycelium and new sclerotia are visible. They fall off the plant onto the soil and thus increase the pathogen density in the soil for the next crop. The mycelium grows in a typical way, with branching at 90 degree angles. This feature makes the fungus more easily identifiable under a microscope.
Within the fungus, 12 different ‘anastomosis groups (AG’s)’ are distinguished. When two hyphae of Thanatephorus cucumeris meet, two different reactions are possible. Hyphae from the same AG fuse. When hyphae from different AG’s meet, the cells around the fusion site die. To complicate matters further, subgroups can be distinguished within AG’s. The different AG’s are more or less specific to the pathogenicity of different host groups.
Not all isolates are pathogenic, many isolates are purely saprophytic soil inhabitants.
